Excitation-revealed changes in cytoplasmic Cl− concentration in “Cl−-starved”Chara cells

Long-distance electrical signals as a link between the local action of stressors and the systemic physiological responses in higher plants

Our review is devoted to the analysis of the role of long-distance electrical signals in the development of the fast systemic physiological responses in higher plants. The characteristics and mechanisms of basic electrical signals (variation potential, action potential and system potential) are analyzed, and a potential schema of the generation and propagation of the system potential is proposed. The review summarizes the physiological changes induced by the variation potential, action potential and system potential in higher plants, including changes in gene expressions, the production of phytohormones, photosynthesis, phloem mass-flow, respiration, ATP content, transpiration and plant growth. Potential mechanisms of the changes are analyzed. Finally, a hypothetical schema, which describes a hierarchy of the variation potential, action potential and system potential, in the development of the fast systemic non-specific adaptation of plants to stressors, is proposed.

Mathematical Models of Electrical Activity in Plants

Electrical activity plays an important role in plant life; in particular, electrical responses can participate in the reception of the action of stressors (local electrical responses and oscillations) and signal transduction into unstimulated parts of the plant (action potential, variation potential and system potential). Understanding the mechanisms of electrical responses and subsequent changes in physiological processes and the prediction of plant responses to stressors requires the elaboration of mathematical models of electrical activity in plant organisms. Our review describes approaches to the simulation of plant electrogenesis and summarizes current models of electrical activity in these organisms. It is shown that there are numerous models of the generation of electrical responses, which are based on various descriptions (from modifications of the classical Hodgkin-Huxley model to detailed models, which consider ion transporters, regulatory processes, buffers, etc.). A moderate number of works simulate the propagation of electrical signals using equivalent electrical circuits, systems of excitable elements with local electrical coupling and descriptions of chemical signal propagation. The transmission of signals from a plasma membrane to intracellular compartments (endoplasmic reticulum, vacuole) during the generation of electrical responses is much less modelled. Finally, only a few works simulate plant physiological changes that are connected with electrical responses or investigate the inverse problem: reconstruction of the type and parameters of stimuli through the analysis of electrical responses. In the conclusion of the review, we discuss future perspectives on the simulation of electrical activity in plants.

Action potential in charophytes

The plant action potential (AP) has been studied for more than half a century. The experimental system was provided mainly by the large charophyte cells, which allowed insertion of early large electrodes, manipulation of cell compartments, and inside and outside media. These early experiments were inspired by the Hodgkin and Huxley (HH) work on the squid axon and its voltage clamp techniques. Later, the patch clamping technique provided information about the ion transporters underlying the excitation transient. The initial models were also influenced by the HH picture of the animal AP. At the turn of the century, the paradigm of the charophyte AP shifted to include several chemical reactions, second messenger-activated channel, and calcium ion liberation from internal stores. Many aspects of this new model await further clarification. The role of the AP in plant movements, wound signaling, and turgor regulation is now well documented. Involvement in invasion by pathogens, chilling injury, light, and gravity sensing are under investigation.

Cl - channels in Chara

The action potential in the cells of the freshwater alga Chara corallina is slower than that in the nerve by about 1000-fold. The depolarization phase is brought on by the outflow of the Cl - ions. Voltage-clamp studies show that this Cl - current can be described by the Hodgkin-Huxley equations for the Na+ transient in the squid axon. The only change necessary to the form of the Hodgkin-Huxley equations is an introduction of a time delay between the stimulus and the onset of excitation. This mathematical model of the Chara action potential facilitates a quantitative description of the effects of pH and temperature. While a pH shift alters various Hodgkin-Huxley parameters, temperature change influences mainly the activation and inactivation time constants but leaves the voltage-dependence of these parameters unaffected. The delays in excitation are both temperature and potential dependent. In future some corrections to the Hodgkin-Huxley picture of the Chara action potential may be necessary, as recent impedance measurements suggest a change in the membrane capacitance at the time of excitation.

Proton/chloride cotransport in Chara: mechanism of enhanced influx after rapid external acidification

Rapid lowering of the external pH ("pH jump") enhances Cl(-) influx in Chara. Experiments were conducted to distinguish between two factors which have previously been proposed to mediate in the response: raised cytoplasmic pH and lowered cytoplasmic Cl(-) concentration. It is concluded that the latter alternative is more likely because: i) Cl(-) influx is reduced at high external pH; ii) influx following the pH jump is never greater than that following pretreatment in Cl(-)-free solution, which reduces cytoplasmic Cl(-) concentration ("Cl(-) starvation"); iii) the joint application of pH jump and Cl(-) starvation does not result in a greater Cl(-) influx than does Cl(-) starvation alone; and iv) addition of NH 4 (+) , which increases cytoplasmic pH, does generate an additional stimulation of Cl(-) influx following a pH jump. It is suggested that the increased cytoplasmic pH at the end of pretreatment at high external pH decays rapidly during the pH jump, and thus any effect on Cl(-) influx is so transient as to be undetectable by the methods used. The results are discussed in terms of a reaction kinetic model for 2H(+)/Cl(-) cotransport (Sanders, D. and Hansen, U.-P, 1981, J. Member. Biol. 58, 139-153) which describes quantitatively; i) the effects of NH 4 (+) on Cl(-) influx in terms involving only a change in cytoplasmic pH; and ii) the combined effects of Cl(-) starvation and NH 4 (+) in terms involving only changes in Cl(-) concentration and cytoplasmic pH. Conversely, the combined effects of Cl(-) starvation and pH jump cannot be described by the model if the effect of the pH jump is the consequence of increased cytoplasmic pH. The simple interpretation of experiments on whole cells involving manipulation of [Formula: see text] (the electrochemical potential difference for protons across the plasma membrane) is questioned in the light of these and previous findings that secondary factors can determine the response of Cl(-) transport in Chara.